1. Field of Invention
This invention relates generally to signal recovery from an array of charge coupled devices, and more particularly, to an apparatus and method for eliminating the need for multiple noise reduction channels for signal recovery from charge coupled devices having multi-phase outputs.
2. Description of the Prior Art
Charge coupled devices (CCDs) can be used to form the light sensitive portion of a television camera. Such applications have been discussed in the literature widely, for example see "Charge Transfer Devices" by Sequin and Tompsett, Academic Press, Inc. 1975. In the television camera, a system of lenses focuses the light associated with the image onto CCDs. The CCDs converts light impulses into electrical impulses representative of the brightness of the image. The electrical impulses are generated for each particular location of each CCD element. The stream of electrical impulses emanating from the CCD is subsequently manipulated to transmit a serial train of electrical pulses descriptive of the image to which the CCDs were originally exposed via the system of lenses of the television camera.
A major problem involved with the operation of CCDs results from the contamination of tile output signal with clock feedthrough and several forms of noise. Clock feedthrough causes frequency components of the clock signals to appear in the output signal. Although, strictly speaking, clock feedthrough is not noise since it is a deterministic process, for present purposes, all sources of signal contamination including clock feedthrough will be referred to as noise. Other types of noise present in the output signal of a charge transfer device are so-called "l/f" noise characterized by an additive noise with high power at lower frequencies and lower power at high frequencies of a signal, and reset noise from the charging of the floating diffusion capacity.
Several schemes have been suggested for reducing or removing noise from the output signal produced by a CCD. These schemes involve some method for separately generating a "noise" signal that is similar and preferably identical to the noise present in the output signal from the device. The separately generated noise signal is then subtracted from the output signal of the device to yield a relatively noise-free output signal.
An example of a noise reduction method is the reflection-delayed noise suppression (RDS) method. This method can, with a delay line alone, determine the difference between the feedthrough level and the signal level of the CCD output signal, detect the signal component, and suppress the noise component. Another method for noise reduction similar to the RDS method is the DDS method. Here, a switch applies clamp pulses at the proper time to reduce noise.
Because of the large number of pixels required by HDTV, many CCDs currently have two phase outputs. A problem with both the RDS and DDS methods is that each CCD output related to each phase of a two phase, or multiphase, CCD must be processed independently through a separate and duplicative set of parts making up the RDS or DDS noise reduction circuits. For example, DDS noise suppression for a two phase CCD requires two buffers, two delay lines, two differential amplifiers, and two sample and hold circuits. The RDS parts count for a two phase CCD is similarly duplicative. This requirement of duplicate parts for each phase of the CCD requires the use of a large number of parts to accomplish the noise reduction function associated with the RDS and DDS methods with multiphase CCDs. A typical circuit of the prior art showing the use of the RDS method with a CCD having two phases is shown in FIG. 1.
It is therefore an objective of the present invention to perform a noise reduction function on multiple phased outputs of CCD devices using a minimal number of components.
It is a further objective of the present invention to increase the reliability of noise reduction circuitry by reducing overall parts count associated with that function.
These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.